1. Field of the Invention
The present invention relates to image forming apparatuses, such as copying machines and laser printers, that adopt an intermediate transfer system of an electrophotographic system or an electrostatic recording system for transferring a toner image formed on an image bearing member onto an intermediate transfer member and thereafter transferring the toner image onto a transfer material.
2. Description of the Related Art
A known example of image forming apparatuses, such as copying machines and laser printers, uses an intermediate transfer member.
An image forming apparatus configured to use an intermediate transfer member transfers a toner image formed on the surface of a photosensitive drum serving as a first image bearing member onto an intermediate transfer member in a primary transfer process. Thereafter, by repeating this primary transfer process for a plurality of colors of toner images, the image forming apparatus forms the plurality of colors of toner images on the surface of the intermediate transfer member. Subsequently, as a secondary transfer process, the image forming apparatus transfers the plurality of colors of toner images formed on the surface of the intermediate transfer member onto a transfer material in a batch. The unfixed toner images transferred in a batch on the transfer material are thereafter fixed permanently by the fixing unit to form a full-color image on the transfer material.
At that time, part of the toner images are not sometimes transferred to the transfer material in the secondary transfer process and thus remains on the surface of the intermediate transfer member. By collecting the residual toner by a known cleaning unit, the next image formation can be started.
Japanese Patent Laid-Open No. 9-50167 discloses an image forming apparatus that collects residual toner on the intermediate transfer member after the secondary transfer process from the intermediate transfer member using a charging unit. This proposes a simultaneous transfer cleaning system in which an AC voltage is applied to a roller used as the charging unit to charge the residual toner to a polarity opposite to the charged state of the toner during development, and the residual toner charged to the opposite polarity is thereafter reversely transferred to a photosensitive drum in the next primary transfer process and is collected by a cleaning unit on the photosensitive drum. The above configuration allows the residual toner to be cleaned simultaneously with the primary transfer of the next page, thus allowing continuous image formation without slowing the printing speed.
Japanese Patent Laid-Open No. 2009-205012 discloses a method of using a roller member and a brush member as a charging unit. Specifically, this is configured to scatter residual toner on an intermediate transfer member substantially uniformly with the brush member and to charge the substantially uniformly scattered residual toner with the roller member. However, the use of the brush member as the charging unit may pose the following problem depending on the situation; that is, conductive fibers that constitute the brush member may cause electric discharge that causes a bad quality image. Specifically, an image forming apparatus in which toner is negatively charged during development will be described.
The brush member described above scatters residual toner substantially uniformly by coming into contact with the intermediate transfer member and charges the residual toner to a positive polarity opposite to the charged state of the toner during development when a DC voltage is applied. As shown in FIG. 6A, the brush member 60 is provided with a predetermined amount of entry with respect to the intermediate transfer member 61. Furthermore, the brush member 60 is connected to a voltage application unit (not shown) that applies a positive-polarity voltage. Therefore, conductive fibers 62 that constitute the brush member 60 are bent into contact with the surface of the intermediate transfer member 61 to form a minute gap L to or from the intermediate transfer member 61. At that time, a large number of minute gaps L are generated between the surface of the intermediate transfer member 62 and the conductive fibers 62, as shown in FIG. 6B that is an enlarge view of a contact portion S at which the intermediate transfer member 61 and the conductive fibers 62 contact in FIG. 6A.
FIG. 7 illustrates a cross-sectional view of one of the conductive fibers 62 constituting the brush member 60 over which a conductive agent is dispersed. Since the whole outer circumferential surfaces of the conductive fibers 62 are covered with the scattered conductive agent, the electric conductive portions of the conductive fibers 62 and the intermediate transfer member 61 oppose each other to discharge electricity in all the minute gaps L. This provides discharging points corresponding to the number of the conductive fibers 62 (minute gaps L in which electric discharge occurs).
As a result, residual toner that passes through the charging portion that the brush member 60 forms is overcharged at a positive polarity (opposite polarity to the charged state of the toner during development) at the large number of charging points formed between the brush member 60 and the intermediate transfer member 61, resulting in an excessive charge amount. When the overcharged residual toner is reversely transferred from the intermediate transfer member to the photosensitive drum at the primary transfer portion, the residual toner is reversely transferred to the photosensitive drum while drawing the negative-polarity toner developed on the photosensitive drum because of a large electric field generated in the surrounding, thus causing a bad quality image.
The above tendency is notable under a high-temperature, high-humidity environment in which the charge polarity of the residual toner before coming into contact with the brush member 60 tends to become opposite to the polarity during development. Since the toner itself absorbs moisture under the high-temperature, high-humidity environment, the resistance is low, so that the absolute value of the charge amount of the toner is small. The charge polarity of the residual toner is reversed due to the influence of the positive-polarity voltage received during the secondary transfer, which increases the proportion of positive-polarity toner, so that the foregoing phenomenon is prone to occur.
To reduce overcharging of the residual toner, the number of minute gaps L formed between the conductive fibers 62 constituting the brush member 60 and the intermediate transfer member 61 should be reduced. To reduce the number of minute gaps L, there is a method of reducing the number of points of contact between the residual toner and the conductive fibers 62 by decreasing the density of the conductive fibers 62 to reduce the number of the conductive fibers 62.
However, this method reduces the points of contact between the conductive fibers 62 constituting the brush member 60 and the residual toner, thus resulting in a decrease in the effect of scattering the residual toner. In particular, if there is much residual toner, lumps of residual toner cannot be scattered by the brush member 60 in which the scattering effect is reduced. This excessively reduces the charge amount of the residual toner after it passes through the contact portion between the brush member 60 and the intermediate transfer member 61. As a result, the insufficiently charged residual toner remains on the intermediate transfer member 61 when reversely transferred to the photosensitive drum from the intermediate transfer member 61 in the primary transfer portion, which tends to generate a bad quality image.
The above tendency is notable under a low-temperature, low-humidity environment in which the charge polarity of the residual toner hardly becomes positive. Since the electrical resistance of the toner itself is high, so that the absolute value of the charge amount of the toner during development is large during development under the low-temperature, low-humidity environment, which increases the proportion of negative-polarity residual toner, so that the foregoing phenomenon is prone to occur.